1. Field
This invention relates to shotcrete application systems and methods. More particularly, this invention provides an apparatus for and a method of injecting shotcrete accelerator into flowing shotcrete within the applicator of a shotcrete application system.
2. State of the Art
It is well known that one of the expanding uses of concrete is in the form of shotcrete. That is, a concrete slurry under pressure is applied to surfaces through a nozzle. Such surfaces may be vertical (e.g., walls) as well as ceiling surfaces. For example, in some circumstances it is desirable to coat the inside of mine tunnels and shafts with shotcrete. So that the concrete (shotcrete) will dry rapidly and adhere to the ceiling and wall surfaces upon which it is sprayed before dripping occurs, a chemical accelerator is mixed into the concrete. The accelerator decreases the set time for the concrete and tends to reduce rebound (splatter) of the shotcrete from the surface upon which it is sprayed.
A wide variety of techniques have been devised to mix the shotcrete accelerator into the shotcrete slurry before application. However, by virtue of the fact that the accelerator decreases the set time (i.e., drying time), the accelerator cannot be mixed into the shotcrete until just before its use. Practically, it is frequently difficult to do so. Yet, mixing the accelerator into the shotcrete slurry too soon will result in a hardening shotcrete slurry which may under certain circumstances become useless because it cannot be sprayed through the nozzle by the virtue of its increased viscosity. Other techniques, including mixing the shotcrete accelerator by mechanical means just prior to or during application, have proven unsatisfactory because of incomplete mixing. Similarly, injecting shotcrete accelerator directly into the flowing shotcrete at some point between its source and its applicator has also proved unsatisfactory because of incomplete mixing. When accelerator is incompletely mixed in with the concrete, portions of the sprayed shotcrete containing a high density of accelerator tend to dry very rapidly while adjoining portions which contain little accelerator drip and run. The shotcrete rebound phenomenon is inevitably more pronounced when the accelerator is incompletely mixed with the concrete.